The present invention relates to a semiconductor device, and more particularly to a resistor having a high resistance value in a monolithic integrated circuit (hereinafter called a "monolithic IC").
A resistor in a monolithic IC, for instance, in a bipolar monolithic IC comprising circuit elements formed in an N-type epitaxial layer on a P-type substrate, is normally constructed of a P-type strip-shaped region formed in the N-type epitaxial layer simultaneously with formation of a base region of a NPN transistor. The sheet resistance and junction depth of the strip-shaped region are uniquely determined depending upon a characteristic of the NPN transistor. Therefore, a predetermined resistance value is realized by adjusting the width and/or length of the strip-shaped region. However, the sheet resistance of the strip-shaped region is low. For this reason, in order to obtain a resistor having a high resistance value, the length of the strip-shaped region would become very long. As a result, the area occupied by the strip-shaped region would become very large.
For the purpose of realizing a resistor having a high resistance value without increasing the occupation are, the so-called pinch resistor and ion-implanted resistor have been used. These resistors have been employed in MOS-type monolithic ICs. The pinch resistor is such that an N.sup.+ -type region is formed on the P-type strip-shaped resistor region so as to overlap with a part of the P-type strip-shaped resistor region and to connect with the N-type region in which the P-type resistor region has been formed, and electrical connection is provided at both ends of the strip-shaped resistor region. The thickness of the strip-shaped region is partially reduced by the overlapping N.sup.+ -type region, and moreover, the portion having a relatively high impurity concentration along the surface of the strip-shaped region does not substantially contribute to the determination of the resistance value. As a result, the resistance value is substantially determined by the sheet resistance of a P-type portion under the overlapping N.sup.+ -type region and the width and length of this overlapped P-type portion. The sheet resistance of the P-type portion is increased by about one order of magnitude. Thus, a resistor having a high resistance value can be realized without increasing the occupation area of the resistor.
On the other hand, the ion-implanted resistor is constituted by an ion-implanted region having a high sheet resistance, i.e., by a strip-shaped region formed by implanting P-type impurity ions into a surface portion of an N-type region at a low concentration. Since the sheet resistance of the ion-implanted region is relatively high, good ohmic contact between an electrode and the ion-implanted region cannot be realized. Hence, contact regions having a low sheet resistance are formed at both ends of the strip-shaped region, and electrodes for the ion-implanted resistor are formed on the contact regions. Accordingly, the resistance value of the ion-implanted resistor is substantially determined by the portion of the ion-implanted region that is located between the contact regions. The formation of the ion-implanted region by the ion-implantation of an impurity is effected before the formation of the electrodes but after active regions for circuit elements have been formed. If heat treatments for forming the active regions are effected after ion-implantation of impurities, the implanted impurities would be rediffused by the heat treatments, so that a desired high sheet resistance could not be obtained. Due to the fact that the ion implantation step is effected after all active regions have been formed as described above, a thickness of a surface insulating film on the ion-implanted region is extremely thin as compared to that of the surface insulating film on its surrounding region which has been formed through several heat treatments included in the diffusion steps effected before the ion implantation step. In other words, the difference in thickness between the surface insulating film on the ion-implanted region and that on its surrounding region is considerably large. For this reason, in the event that an interconnection conductor passes over an ion-implanted resistor, it may possibly occur thsat the interconnection conductor is broken at the steps of the insulating films. In order to obviate this shortcoming, a semiconductor region is additionally formed simultaneously with the contact regions in the ion-implanted resistor. The interconnection conductor crosses above this semiconductor region, the insulating film on which is thick during the subsequent steps including the heat treatment. Since this semiconductor region has a low sheet resistance, the ion-implanted region is elongated by the length corresponding to the length of that semiconductor region. Consequently, an ion-implanted resistor over which an interconnection conductor passes includes at least two ion-implanted resistor regions, two contact regions and a semiconductor region having a low sheet resistance for connecting the two ion-implanted resistor regions to each other. This semiconductor region is referred to, hereinafter, as a connecting region.
Where the aforementioned pinch resistors and ion-implanted resistors are used as resistors in a circuit such as, for example, a differential amplifier, a flip-flop or resistance divider circuit which requires a precise resistance ratio, a current flowing through the circuit could be reduced for a low power consumption, and further the design of a desired heat dissipation capacity of a package for sealing a semiconductor chip could become easy. However, with respect to the pinch resistor, undesired expansion of the N.sup.+ -type region overlapping the P-type resistor region which occurs in its formation by diffusion or annealing directly influences the resistance value of the pinch resistor, while in the ion-implanted resistor undesired expansion of the contact regions and the connecting region occurring in their formation directly influence the resistance value of the ion-implanted resistor. More particularly, with the expansion of the N.sup.+ -type region in the pinch resistor, the length of the P-type portion overlapped with the N.sup.+ -type region is undesirably expanded, so that the resistance value of the pinch resistor becomes higher. On the other hand, the expansion of contact regions and the connecting region in the ion-implanted resistor undesirably decreases the length of the ion-implanted resistor region between them, so that the resistance value of the ion-implanted resistor becomes lower. For this reason, it has been almost impossible to realize a precise resistance ratio by the use of pinch resistors or ion-implanted resistors, even if the lengths of the respective resistors are made to have an intended ratio. Where the intended ratio is one, it is difficult to form two resistors in a congruent shape desired from the point of view of a pattern layout including transistors and other elements, and it is impossible to realize the ratio of one with two resistors of different configurations.